More than 95% in number of the chloroplast polypeptides are synthesized in the cytosol. These polypeptides are made as larger precursors which are subsequently imported into the organelle by an ATP-dependent, post-translational mechanism. The precursor chain extension (transit sequence) presumably plays a role in interaction with envelope receptors and facilities entry of the precursor into the chloroplast. The reposed research is directed toward defining the molecular details of the transport, processing, and assembly of this particular group of polypeptides. We will focus on the precursor (pS) to the small subunit of ribulose bisphosphate carboxylase and the precursors (p15 and p16) to the polypeptide subunits of the chlorophyll a/b protein complex. A wheat germ translation mix containing the precursors will be fractionated by density gradient centrifugation and column chromatography to see whether the transport-component form is monomeric or polymeric. pS purified by immunoadsorption will be tested for its ability to enter intact chloroplasts. These experiments will clarify whether polypeptide uptake requires one or more cytosolic protein factors that may be present in the wheat germ extract. To identify transport receptors, intact chloroplasts will be treated with proteases and the digested sample and undigested control will be compared in terms of their uptake efficiency and their envelope polypeptide profiles. Inhibition of uptake of specific precursors will be correlated with cleavage of specific envelope polypeptides. Immunochemical experiments using Fab fragments of monospecific IgG raised against the protease-sensitive envelope polypeptides will aid in the identification of transport receptors. cDNA clones encoding pS and p15 will be sequenced to elucidate the amino acid sequence of the precursor transit peptides.